[llvm-reduce] Count chunks by running a preliminary reduction

Having a separate counting method runs the risk of a mismatch between
the actual reduction method and the counting method.

Instead, create an Oracle that always returns true for shouldKeep(), run
the reduction, and count how many times shouldKeep() was called. The
module should not be modified if shouldKeep() always returns true.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D113537